Heat treatments of die castings under pressure

ABSTRACT

METALLIC DIE CASTINGS OF MAGNESIUM, ALUMINUM AND ALLOYS THEREOF ARE SUBJECTED TO A HEAT TREATMENT OF FROM 300* TO 600*C. UNDER A PRESSURE OF FROM 100 TO 10,000 P.S.I. FOLLOWED BY A LOW TEMPERATURE AGING TREATMENT TO IMPROVE THE PROPERTIES OF THE METALS WITHOUT FORMING BLISTERS ON THE DIE CAST PRODUCT.

United States Patent 3,732,128 HEAT TREATMENTS F DIE CASTINGS UNDERPRESSURE Charles D. Statham, Trenton, NJ., assignor to N L Industries,Inc., New York, N .Y. No Drawing. Filed July 16, 1971, Ser. No. 163,480Int. Cl. C22f 1/04, 1/06 US. Cl. 148159 3 Claims ABSTRACT OF THEDISCLOSURE Metallic die castings of magnesium, aluminum and alloysthereof are subjected to a heat treatment of from 300 to 600 C. under apressure of from 100 to 10,000 p.s.i. followed by a low temperatureaging treatment to improve the properties of the metals without formingblisters on the die cast product.

BACKGROUND OF THE INVENTION High temperature heat treatments are usuallynot employed with dies castings. Temperature treatments of 300 to 500 C.are sometimes used with sand and permanent mold castings or on wroughtproducts. In these treatments, however, only atmospheric pressure isused. The treated metal or alloy is then usually subjected to a lowtemperature heat treatment of 100 C. to 250 C. to improve the propertiesof the metal or alloy.

Magnesium and aluminum metal and alloy die castings normally cannot betreated at temperatures of 300 C. to 500 C. without forming blisters onthe surface of the metal or alloy.

SUMMARY OF THE INVENTION A method for improving the mechanicalproperties of a metallic die casting selected from the group consistingof magnesium and aluminum and alloys thereof which comprises placing thedie casting in a closed container, heating the die casting up to 300 C.to 600 C. under a pressure of from 100-10,000 p.s.i. for 1-72 hours andrapidly cooling said die casting while still maintaining the appliedpressure. This heat treated die casting is then subjected to a lowtemperature aging treatment of from 100 C. to 250 C. for 1-72 hours atatmospheric pressure. These two heat treatments in combination improvethe mechanical strength of die casting.

DESCRIPTION OF PREFERRED EMBODIMENTS Both magnesium and aluminum metaland alloys thereof when subjected to a die casting operation form a diecasting which is porous. This porous metal or alloy normally cannot beheated to a temperature of 300 C. to 600 C. because blisters form on themetallic surface. Blisters begin to form at about 300 C. for magnesiumand about 350 C. for aluminum.

It has now been discovered that the porous die casting may be heated at300 C. to 600 C. without forming blisters if pressure of 100-10,000p.s.i. is employed during heat treatment.

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This high temperature treatment followed by the low temperature heattreatment produces a die casting with increased mechanical strengthproperties which heretofore have not been obtained.

In order to describe the instant invention in more detail the followingexamples are presented:

Example I In this example an aluminum alloy die casting weighing 200grns. was placed in an autoclave. The die casting used wasA.S.T.M.-SC84A. The die casting was heated to 480 C. at 2000 p.s.i.pressure for four hours.

After the heat treatment, the die casting was cooled rapidly and thepressure was released. This die casting was then subjected to lowtemperature heat treatment of 175 C. for eight hours at atmosphericpressure.

The final die casting after high temperature heat treatment underpressure followed by the low temperature heat treatment was examined forits mechanical strength properties.

It was found that its yield strength was 50 k.s.i. compared to 25 k.s.i.for the untreated alloy die casting. This pressure treated die castingwas substantially free from blisters due to the use of the pressure heattreatment.

Example II In this run the procedure of Example I was repeated exceptthat the aluminum alloy die casting used was A.S.T.M.-SN122A.

After the die casting was subjected to the pressure heat treatmentfollowed by the low temperature treatment, the treated die casing wassubstantially free from blisters. The yield strength of the treated diecasting was 60 k.s.i. compared with only 35 k.s.i. for the untreated diecasting.

Example III In this run a magnesium alloy die casting was used in placeof an aluminum alloy. The magnesium alloy die casting was identified asA.'S.T.M.-AZ91B. This particular die casting was pressure heat treatedat 400 C. for 20 hours and rapidly cooled followed by the lowtemperature heat treatment at 175 C. for 24 hours at atmosphericpressure.

Again after subjecting the die casting to the high temperature andpressure heat treatment followed by the low temperature heat treatment,the final die casting had a greater yield strength than the untreateddie casting.

While this invention has been described and illustrated by the examplesshown, it is not intended to be strictly limited thereto, and othervariations and modifications may be employed within the scope of thefollowing claims:

What is claimed is:

1. A method for treating a metallic die casting selected from the groupconsisting of a casting of aluminum, magnesium and an alloy thereof, toimprove the mechanical properties of said die casting which comprisesplacing said diecasting in a closed chamber, heating said casting at atemperature from 300 C. to 600 C. under a pressure of from to 10,000p.s.i. for a period from 1 to 72 hours and after said treatment, rapidlycooling said casting while still under pressure, and then heating saidcasting of a temperature of from 100 C. to 250 C. for 1 to 72 1,891,23412/1932 Langenberg 148-131 X hours at atmospheric pressure andsubsequently cooling 3,156,974 11/1964 Bobrowsky 1484 X said casting toroom temperature. 3,157,540 11/1964 Bobrowsky 148-431 2. Methodaccording to claim 1 in Which said die cast- 3,329,535 7/1967 Langer eta1. 1484 ing is an alloy of aluminum. 5

3. Method according to claim 1 in which said die cast- OTHER REFERENCESing is an alloy of magnesium. Alloy Digest, Filing Code Mg-36, Engr.Digest Inc.,

New Jersey, January 1958. References Cited UNITED STATES PATENTS 10CHARLES N. LOVELL, Primary Examiner 3,496,624 2/1970 Kerr 1484 X US. Cl.X.R. 1,946,545 2/1934 Pcssel 148131 148131, 161

